[InstructionCost] Add saturation support.

This patch makes the operations on InstructionCost saturate, so that when
costs are accumulated they saturate to <max value>.

One of the compelling reasons for wanting to have saturation support
is because in various places, arbitrary values are used to represent
a 'high' cost, but when accumulating the cost of some set of operations
or a loop, overflow is not taken into account, which may lead to unexpected
results. By defining the operations to saturate, we can express the cost
of something 'very expensive' as InstructionCost::getMax().

Reviewed By: kparzysz, dmgreen

Differential Revision: https://reviews.llvm.org/D105108